Thin-film hard disc magnetic media are widely used for on-line data storage in computers. In recent years, considerable effort has been devoted to achieving higher areal recording density. Higher recording densities can be achieved using recording heads with narrower trackwidth and reduced gap (Futamoto; Tsang), high linear resolution, low noise, and high signal-to-noise ratio (SNR) (Yogi), and reduced flying height, i.e., spacing between the flying head and media (White).
Several approaches for reducing noise in thin-film recording media, for increasing disc performance and areal recording density, have been reported. In general, these approaches are aimed at achieving greater grain isolation in the magnetic layer, to reduce inter-grain exchange coupling (Hata). One approach to increasing grain isolation is based on compositional segregation. Reduced noise can be achieved, for example, in a Co-based alloy, such as Co/Cr/Ta, where non-magnetic alloy components, such as Cr and Ta, serve to isolate the magnetic grains in the magnetic layer.
Another approach for reducing media noise is based on physical grain separation, and uses process conditions and layer thickness to increase grain isolation in a magnetic layer. Higher sputter pressure, for example, is known to lead to greater grain isolation in a magnetic layer. Increasing Cr underlayer thickness, in the range above about 2,000 .ANG., reduces media noise, apparently by increasing grain isolation in a magnetic film sputtered over the underlayer. In general, grain isolation can be expected to decrease with decreasing layer thickness, and several reports have shown that media formed with a Co-based magnetic layer exhibit lower noise as the thickness of the magnetic layer decreases (Lambert, Sanders). The reduction in media noise was accompanied by a reduction in coercive squareness, which is associated with intergrain coupling (Hata).
A limitation in reducing magnetic film thickness, in reducing media noise, is that the amplitude of the read-back signal also falls with reduced film thickness, limiting the signal-to-noise ratio achievable with a thin magnetic layer. One solution to this problem, which has been described in several reports, is to divide the magnetic layer into two or more thinner layers separated by nonmagnetic isolation sublayers. The multilayered configuration preserves the low noise characteristics of the thin, isolated layers, but has signal amplitude characteristics related to the combined thicknesses of the magnetic layers.
The multilayered configuration thus combines low noise and high signal-to-noise characteristics. In addition, high media coercivity can be achieved with a multilayered configuration. However, commercial production of such media has been limited heretofore by the additional target-materials expense, processing steps, and sputtering-machine modifications needed to produce a multilayered medium.